The present invention relates to a measuring machine, particularly of the portal type.
It is known that portal measuring machines include a base defining a horizontal reference plane adapted to support a body to be measured in a fixed manner, a portal movable on the base along a first horizontal axis (X) and having a cross-member extending along a second horizontal axis (Y) perpendicular to the first, a carriage movable on the cross-member along the Y axis and a measuring head carried by the carriage and movable relative thereto along a third, vertical axis (Z). The head carries a tracer point at its lower free end.
Generally the carriage is supported for sliding movement on the cross-member by a plurality of pneumatic bearing pads which cooperate with respective slide tracks for the bearing pads themselves, formed on the cross-member. The arrangement of the slide surfaces and of the bearing pads is such as to enable the carriage to slide along the Y axis with the minimum friction but to prevent any other translational movement (along the X and Z axes) and rotational movement (about any one of the three axes X, Y and Z).
In known measuring machines, the slide surfaces are formed on the cross-member in positions which define a main slide plane for the carriage which is parallel to the YZ plane or to the XY plane, and the slide surfaces are sufficiently spaced from each other in this plane as to optimise the torsional rigidity about the Y axis; hence, the cross-member and the carriage generally have cross-sections which are elongate in directions parallel to the Z axis or to the X axis. The slide surfaces may be perpendicular to the X and Z coordinate axes or, more conveniently, formed on a guide of dovetail shape.
These known configurations involve a certain number of problems.
First of all, when the cross-section of the carriage is elongate in the Z direction, torsional rigidity about the X axis is generally poor; it is, however, desirable for this rigidity to be sufficiently high (as in the case of the rigidity to rotations about the Y axis) since the distance of the tracer point from the centre of gravity of the carriage amplifies the rotational movements to a greater extent the more the head projects and hence introduces errors into the measurements.
A further problem connected with the elongate form in the Z direction is a greater sensitivity to thermal gradients which, under normal conditions, are observed mainly in the vertical direction; different regions of the carriage are thus subject to differing thermal expansions which introduce errors into the measurements or at least must be compensated for by processing of the measurements detected.
In known solutions in which the cross-member and the carriage have elongate sections in the direction of the X axis, these problems are reduced but others present themselves.
In particular, the rigidity of the cross-member against bending in the YZ plane is somewhat reduced with the obvious disadvantages this involves; moreover, the overall bulk of the carriage in the X direction, including that part of the carriage which houses and guides the head and which must, therefore, project outwardly of the transverse bulk of the cross-member, is excessive and this, in addition to being disadvantageous in itself, causes a very disadvantageous mass distribution from the inertial point of view: a high moment of inertia involves inertial couples of considerable magnitude, particularly in view of the strong accelerations to which the carriage is subject in use.
A further disadvantage of known configurations, which all have upwardly facing horizontal slide surfaces, is the accumulation of dust and dirt in general under gravity which therefore necessitates frequent maintenance.